The reason I asked is because there are lots of places from which one could launch where few/none of those 7 billion could witness it, if one were willing to put in the effort and money. The US military has a history of doing such things (Enewetak, et c).

Obviously such locations are constrained by orbital considerations, but also there is FH which may be able to mitigate some of that if money were little/no object. We already know SpaceX has people with clearances that work with classified payloads semi-regularly.

Stealth satellites are also a thing. I'm not sure that I am fully up to date on the state of the art here, or that being such is even possible outside of national military efforts.

Could you elaborate on these sensors you speak of? I know there are ground-based optical tracking systems in use, but the satellite-based surveillance I have heard of is mostly to detect nuclear explosions, not rocket launches themselves.